This invention pertains to lubricants and, more particularly, to lubricating grease thickener systems.
Early lubricating greases were thickened by metal soap salts of fatty acids. Metals commonly used were sodium, calcium, aluminum, and lithium. Other metals have also been used, but with less frequency. Fatty acids included various vegetable and animal fatty acids as well as those derived from petroleum sources. In recent years, the preferred fatty acids have been hydroxylated stearic acids, most preferably 12-hydroxystearic acid.
Soaps for greases were commonly provided by reacting the metal hydroxide, oxide, carbonate, or other metallic basic compound with the fatty acid to form the corresponding fatty acid soap. This thickener formation reaction usually occurred directly in the base oil which was to be thickened. Depending on the thickener being formed, water was often used as a reaction solvent or stabilizer. If a fatty acid derivative such as an ester was used as the source, water was added to hydrolyze the derivative and free the fatty acid which could then react with the basic reagent to form the fatty acid soap. If water was not required in the final product to stabilize the thickener system, the water was generally removed by heating the grease above 212.degree. F.
Such fatty acid soap thickeners have been used for many years and are often referred to as simple soap thickeners. Depending on the metallic base used, greases thickened by such simple soaps have dropping points of 200.degree. F. to about 380.degree. F. Traditionally, the dropping point of these simple soap thickened products defined the highest operating temperature by the following qualitative relationship: the highest temperature of satisfactory performance is 100.degree. F. less than the dropping point. So, even when using a lithium soap thickened grease with a dropping point near 400.degree. F., the maximum useful operating temperature of that grease was only about 300.degree. F.
As severity of lubricating grease applications increased, the need for thickener systems with higher dropping points became apparent. This gave rise to the development of so-called complex soap thickeners. The complex soap thickeners most commonly used are calcium complex, lithium complex, and aluminum complex.
Lithium complex and aluminum complex greases generally have poor thermal and oxidative stability at sustained high temperatures, such as 350.degree. F. At such temperatures, the grease rapidly degrades to a lacquer-hard material which is devoid of any lubricating properties. This so-called lacquer deposition is often considered to be the result of catastrophic oxidation of the grease and is probably promoted by the lithium complex and aluminum complex thickeners. The use of antioxidants can somewhat delay this occurrence but cannot prevent it. As long as the lithium complex or aluminum complex thickeners are present, lacquer deposition will occur. Despite their higher dropping points, lithium complex and aluminum complex greases are limited in their performance at sustained high temperatures.
Calcium complex thickened greases can also severely harden under sustained high temperatures, although usually not to the lacquer hard condition exhibited by lithium complex and aluminum complex greases. However, calcium complex greases have other problems. Even when stored at 75.degree. F., calcium complex greases will slowly harden when exposed to air. The hardening will begin at the grease/air interface and slowly extend further into the bulk of the grease with time. This phenomenon is well known and is often referred to as skin/age hardening.
The hardening characteristics of complex soap thickened greases can cause a number of problems in actual applications. In bearing applications where the bearing is running only part of the time but experiences high temperatures during those times, such hardening effects can seriously reduce bearing life. In applications where fretting (oscillatory) motions are experienced, long term grease hardening can cause catastrophic failure due to starvation of functional lubricant.
Another problem shared by lithium complex and calcium complex greases is that of reduced thickening power of the thickener. Simple lithium soap greases with an NLGI No. 2 grade consistency will typically have a lithium soap content of 6% to 7% based on the weight of the grease. However, in lithium complex thickened greases of equivalent consistency, nearly twice the amount of thickener, 12% to 14%, based on the weight of the grease, is required. Calcium complex thickened greases show the same behavior. An NLGI No. 2 grade calcium complex grease will typically require soap levels of 16% to 18%, based on the weight of the grease, compared with about 8% for simple calcium soap thickened greases of equal consistency. One problem often associated with these higher thickener levels is that of inferior pumpability.
One thickener system which has been used with significant success as an alternative to the above complex soap thickeners is polyurea. Polyurea thickener has may fine qualities which make it a superior lubricating grease thickener compared with lithium complex, calcium complex, and aluminum complex thickeners. Polyurea does not exhibit high temperature lacquer deposition and generally has acceptable pumpability characteristics. The dropping point of polyurea is above 450.degree. F., and usually near or above 500.degree. F. When polyurea thickened greases exhibit their dropping point, it is due to the thickener's inability to hold the oil and not due to the polyurea melting. This is in contrast to most complex soap thickeners which melt at their dropping points.
In spite of their many favorable attributes, polyurea has several characteristics which have limited its usefulness as a lubricating grease thickener. Polyurea thickened greases retain their original consistency quite well when subjected to high shearing forces, but can soften significantly when subjected to lower shearing forces. For instance, in the 100,000 stroke penetration test, ASTM D217, polyurea greases usually soften by 60 to 100 points or more. Similar softening effects can occur when polyurea greases are subjected to the roll stability test, ASTM D1831. Polyurea thickened greases can also have oil separation characteristics which significantly increase as the temperature increases. This is a characteristic which is also exhibited by many complex soap thickened greases.
Over the years, various types of grease thickener systems and greases have been suggested. Typifying these prior art grease thickener systems and greases are those found in U.S. Pat. Nos. 2,197,263, 2,599,553, 2,898,296, 2,940,930, 3,681,242, 3,791,973, 4,107,058, 4,205,831, 4,297,227, 4,435,299, 4,440,658, 4,444,669, and 4,536,308. These prior art grease thickener systems and greases have met with varying degrees of success, but their performance has generally been limited under a varying range of conditions.
It is, therefore, desirable to provide an improved lubricating grease thickener system which overcomes most, if not all, of the above problems.